Questions tagged [convection]

transport mechanism involving bulk motion of a medium

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3 answers
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If you moved your hand continuously through fire, would it feel hot?

If you quickly pass your finger or hand through a candle or fire, it doesn't feel very hot (great party trick). Is this because your hand spends such little time in the fire that there isn't enough ...
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1 answer
25 views

Equation for Convective Heat Transfer

In this Wikipedia link: https://en.wikipedia.org/wiki/Convection_(heat_transfer)#Convective_heat_transfer on convective heat transfer, the following equation for convection is provided: $$\dot{Q}=hA(T-...
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Can mass transfer occur without a concentration difference? (analogous to the question, can energy transfer occur without a temperature difference?)

The driving force for diffusion of mass is a concentration gradient. However, mass can be transported both in a diffusive and a convective (due to the flow of a fluid) way. By definition, the combined ...
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1 answer
259 views

Building a house that is warm in winter and cool in summer [closed]

The Northern house architecture is strikingly different from that used in the South (I am not taking extreme cases, thinking more of Northern or Germany versus Mediterranean: Thick layers of thermal ...
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1 answer
400 views

Which are the best books I can refer for turbulent heat transfer and its computation?

I am a graduate student. I wanted to do computation on heat transfer problem mainly heat convection. I am working on turbulent regime. Can you kindly suggest some good books so that I can get ...
5 votes
1 answer
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Small parameter expansion to obtain convective heat transfer solution of the heat equation

I'm trying to follow along with this paper on perturbative transport analysis. The section I'm stuck on analyzes the heat equation in 1D slab geometry. To get a diffusion-only solution is relatively ...
10 votes
3 answers
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Physics of the convection oven

I don't understand how convection ovens can cook food faster than radiant heat ovens. Guides to convection ovens claim that they cook food 25-30% faster than radiant heat ovens, but do not really ...
-1 votes
3 answers
233 views

Why don't the "trapped warm air" just fly out of the holes in blankets?

I've read a lot of explanations that the holes of a blanket somehow trap air which keeps the air in place.(including this: https://www.reddit.com/r/explainlikeimfive/comments/10lj0h/...
1 vote
1 answer
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Convective derivative N-S

This is probably an easy answer, but I've not been able to find it yet - Why in some formulations of the N-S equations (for example here https://www.grc.nasa.gov/www/k-12/airplane/nseqs.html), is the $...
1 vote
1 answer
54 views

How does air cool as it nears the poles?

I understand air is heated by the equator causing it to rise towards the poles. But why does air cool and sink after nearing the poles. Shouldn't the air still possess heat, after being heated by the ...
1 vote
1 answer
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Geometrical Rotation and Rotation operator relation and ambiguity

In Cohen-Tannoudji's it is said (vol.1 page 694): $|\psi \rangle$ is the state of the system before the rotation. $|\psi ' \rangle$ is the state of the system after the rotation. $|\psi ' \rangle= R |\...
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1 answer
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What causes the lamp above my gas stove to swing like a pendulum when the stove is on?

There is lamp hanging above my gas stove, which when the stove is turned on performs pendulum-like oscillations in an oval trajectory, not unlike the dynamics of Lissajous patterns. The lamp is ...
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The local and average Nusselt number in a square cavity

I am in the process of programming the local & average Nusselt number in a left vertical wall but my Matlab script gives me inappropriate values and it doesn't change with changing of Rayleigh ...
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188 views

Average Nusselt number and heat transfer coefficient

For the flow over a flat plate of length L, how the average Nusselt Number over a length L is equal to $$Nu_{avg} = \frac{h_{avg}L}{k_f}$$ where $h_{avg}$ is the average heat transfer coefficient over ...
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Gas to solid heating?

I am doing a study on the phenomenon of pyrolysis. This is purely a study of the underlying physics. Generally, in practice, pyrolysis feedstock is heated conductively but we are examining the energy ...
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1 answer
44 views

Convection from rapidly evaporating liquid inside vacuum chamber?

If we have a liquid that is is being actively heated and is rapidly evaporating (let's say around 1g/s for Ag at 1500K) inside a vacuum chamber, can convection around the liquid/vapor interface occur ...
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1 answer
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Is destratification ventilation more efficient at ceiling level?

This question is more of a practical physics question. In buildings with heating systems, heated air tends to rise and create air temperature stratification - hottest air near the ceiling, coldest air ...
4 votes
5 answers
835 views

Can conduction alone cause thermal equilibrium?

From the little I know about physics and conduction specifically, it seems to be that a system in which the heat transfer is done solely by conduction, cannot reach thermal equilibrium. I'll preface ...
1 vote
1 answer
431 views

How to treat Nusselt numbers that are smaller than one?

The Nusselt number is defined as $$\text{Nu}=\frac{h\delta}{k},$$ with $h$ as convection heat transfer coefficient,$\text{W}/(\text{m}^2\text{K})$, $\delta$ as characteristic length, $\text{m}$, and $...
11 votes
1 answer
412 views

A theoretical model for evaporation occurring when wind is blowing over the surface of water

I've recently done an experiment in which I was studying the variation of the cooling rate of hot water in a draft (wind). The air was blown directly over the surface of the water (kept in a ...
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2 answers
116 views

How can the rate of convection up a chimney be computed? [closed]

I have various physics textbooks, but none of them explain exactly how to compute the rate of convection. I assume that Newton's Law of Cooling should be used by I can't figure out how to apply it to ...
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3 answers
36 views

Heat Transfer Conservation in a natural convective flow

In a initially stationary area, suppose I have a heat source of say 10 W. Now if I consider a fictional cube (box)/ boundary of say volume 1 m3 around it. And I sum up the heat across the surfaces of ...
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Spectrum of the background radiation on earth

Suppose we consider an object of temperature $T_o$, whereas the air temperature is $T_a$. The object will exchange heat with its environment via a range of processes: Radiating and absorbing heat ...
3 votes
3 answers
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Will a liquid boil quicker if it is stirred versus if it is not stirred

Will the whole liquid boil quicker on a stove-top if it stirred versus if not stirred? What should be the frequency of stirring to get to boiling point quickly? please explain your hypotheses with ...
1 vote
3 answers
83k views

What is the characteristic length of a cylinder

I have a cold cylinder that is submerged in hot water and I need to find the convective heat transfer coefficient. I can do the whole process but I am stuck finding the characteristic length. I found ...
0 votes
1 answer
141 views

Heat Transfer Rate to Surroundings of Heating Battery

I think this is probably a trivial question, but I am trying to understand how to do a thermodynamic calculation for heat transfer. I want to make a basic resistive heater by shorting a battery, but I ...
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0 answers
37 views

Rayleigh Benard Convection analyzed as heat engine?

The treatment I've seen for Benard convection in a textbook is to derive approximate equations of motion and then show that there is a periodic solution matching the boundary conditions. This is ...
1 vote
1 answer
477 views

How much energy is required to keep water hot?

I have a hot-water jug that keeps water hot (say, at about $70\, ^{\circ} C$). To compute its operation cost, I would like to compute its daily energy consumption. The label on the jug says "800 ...
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2 answers
153 views

How to modify single phase fluid/solid coupled PDEs to account for a phase change?

I have two PDEs that model both fluid and solid temperature change due to fluid flow through a packed bed. Schematic and equations here (where the f and s subscripts are for the fluid and solid ...
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How to calculate convection between a hot and a cold area through a circular hole?

Consider the below problem: where we have two half infinite worlds with $T_2 < T_1$ and a circular hole with the diameter of $D$ in an infinite isolating wall. There is no ...
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2 answers
169 views

Calculate how hot air will get passing source of heat

Is there a way to calculate how warm air will get when it passes a source of heat knowing how fast the air is moving? For example, let's say there is an object being heated by 40W of electricity and ...
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1 answer
135 views

Understanding Convectional Rainfall

Consider the following explanation of convectional rainfall given in a VII grade textbook. When air comes in contact with the hot surface of the earth, it gets heated, becomes light and rises in the ...
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1 answer
113 views

In this scenario, if I continue to increase the length of heat exchanger how will the temperatures change?

Consider a counter flow double pipe heat exchanger inside which the cold fluid enters at 20 degrees Celsius and hot fluid enters at 100 degrees Celsius. Initially the length of the heat exchanger is ...
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1 answer
832 views

Arithmetic and Log mean temperature difference of a double pipe parallel flow heat exchanger

Consider a double pipe parallel flow heat exchanger. For such a heat exchanger the LMTD is given as $$\Delta T _{LM}= \frac{\Delta T_i-\Delta T_e}{ln(\frac{\Delta T_i}{\Delta T_e})} $$ The relation ...
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2 answers
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'Convective' Fourier number

I'm working with a numerical solution to a 1D advection-diffusion equation for a multiphase problem, with convective exchange between the gas and solid phase. A non-dimensional parameter which comes ...
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1 answer
84 views

Entropy Change in Radiation and Convection

When a system loses it's internal thermal energy by emitting radiation or by convection what is change of the systems entropy mathematically? I saw this question's answers for radiation only but still ...
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Is this value of Critical Reynolds Number also valid for non circular ducts?

The flow remains laminar in case of internal flow through pipes up to a Reynolds number of 2300, for most practical cases. Is this value valid only for circular pipes or even for non circular ducts?
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2 answers
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Does the convective heat transfer coefficient depend on temperature difference?

Consider a flat plate at a higher temperature than the fluid which is flowing over it, with some velocity. Forced convection is taking place. I'm convinced that the value of heat transfer coefficient ...
4 votes
4 answers
306 views

Why doesn't convection break conservation of energy?

assumption: Warmer gas rises. (under gravity) assumption: A volume of gas cannot have uniform temperature, because gas with fully uniform temperature (all molecules having the same speed) has lower ...
1 vote
1 answer
92 views

Rayleigh number for long slit heated at the end

The Rayleigh number is generally described as distinguishing between the convective and coductive regime of heat flow in a fluid. I'm looking at a specific geometry. and it's not clear to me precisely ...
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3 answers
42k views

Is heat from a stovetop, transfered through convection, radiation or conduction?

It doesn't appear to be convection, as there are no moving objects (or are there); probably not radiation, so it is conduction? I am confused because it seems that the molecules in the stove does not ...
1 vote
1 answer
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Is there convection between two volumes of air at different temperatures?

As I have learned in my engineering classes convection takes place when a fluid meets a surface of both different temperatures. My question: can this also be evaluated for two volumes of air at ...
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1 answer
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Heat Transfer on boiling water

Let's say I'll boil water on the stove from 25°C to 100°C, what heat transfer happens? Is it conduction (heat warms up the pot), or is it convection (causes water molecules to move away from the ...
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1 answer
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Why do we consider the normal component of velocity in a flux and not the original direction of velocity?

Below is the equation of convective flux of an arbitrary property that is carried by particles. When particles move they carry their property along with them. Given a fixed surface in space, in order ...
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What is the ideal gap between two panes of glass for insulation? [closed]

I have two panes of glass, about a 3/8" thick (.95 cm). Space efficiency and aesthetics ignored, how far apart should they be spaced to maximize heat insulation, assuming the gap contains regular ...
1 vote
1 answer
181 views

While determining LMTD for a heat exchanger do we assume, the tube wall to be at a constant temperature (no variation along radial direction)?

The derivation of Log Mean Temperature Difference (LMTD) for a Heat exchanger at preliminary level makes some assumptions like, constant specific heats, steady state operation, no changes in kinetic ...
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1 answer
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Intuitive explanation of convection in a closed volume

I am trying to wrap my head around what happens when say a hollow cylinder containing a gas is heated from the bottom. Intuitively I know that the gas in the cylinder will heat up, expand and pressure ...
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4 answers
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Realistic calculation of heat loss for pipe

Good day everyone, I am new on this site and I hope to find here help, since I am not going anywhere with the literature I have found. I try to calculate realistically the heat loss of a hot, ...
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Thermal Boundary Layer

When we study convection in Heat Transfer then there a part where a fluid flows over a plate maintained at a temperature T. In this case heat transfer ocurrs both by conduction as well as convection ...
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257 views

Why is the rate of energy conducted in a rod equals the rate of energy released from the surface at the other end?

Assuming energy is released as convection only,we always do questions by equalizing, $$P_{\text{conducting}} = Q/t = KA (T1 - T2)/L = eA(dT).$$ Assume the system is at Steady State. Why does this ...